Thorium is a soft, ductile, lustrous, silver-white, radioactive metal. At ordinary temperatures it has a face-centered cubic crystalline structure. It is a member of the actinide series in Group 3 of the periodic table and is sometimes classed as one of the rare-earth metals. When pure, the metal is stable and resists oxidation, but it is usually contaminated with small amounts of the oxide, which cause it to tarnish rapidly. It reacts slowly with water and is attacked only by hydrochloric acid among the common acids. The finely divided metal readily ignites when heated, burning with a brilliant white flame; the oxide formed has the highest melting point of all oxides. Thorium forms numerous compounds with other elements.

Thorium is widely distributed in small amounts in the earth's crust, being about half as abundant as lead and three times as abundant as uranium. The chief commercial source of thorium is monazite sands obtained from India and Brazil. It is also found in the minerals thorite (thorium silicate, ThSiO4) and thorianite (mixed thorium and uranium oxides). Vast deposits of low-grade thorium ore in New Hampshire are a potential source. Thorium metal is isolated with difficulty; it is obtained from certain of its compounds by electrolysis or by chemical reduction. Thorium is used in magnesium alloys and in tungsten filaments for light bulbs and electronic tubes. The most important thorium compound is the oxide (thoria, ThO2), which is the major incandescent component of the Welsbach mantle; it is also used in crucibles, in special highly refractive optical glass, and in catalysts for several industrially important chemical reactions. Important uses of the element result from its natural radioactivity.

There are 26 known radioactive isotopes, only 12 of which have half-lives greater than 1 sec. The most stable is thorium-232 (half-life 1.40 × 1010 years); it is the major component of naturally occurring thorium. Thorium-232 undergoes natural disintegration and eventually is converted through a 10-step chain of isotopes to lead-208, a stable isotope; alpha and beta particles are emitted during this decay. One intermediate product is the gas radon-220, also called thorium emanation or thoron. Thorium and its decay products are sometimes used in radiotherapy. Although thorium-232 is not itself a nuclear reactor fuel since it will not sustain a chain reaction, it may be converted into the fissionable fuel uranium-233, but uranium-233 has not proven to be a practical alternative to natural uranium.

Thorium-232 can react with a thermal (slow) neutron to form thorium-233, emitting a gamma ray. Thorium-233 decays (half-life about 22 min) to protactinium-233, emitting a beta particle. The protactinium-233 decays (half-life about 27 days) with another beta particle emission to uranium-233. Fission of the uranium-233 can provide neutrons to start the cycle again. This cycle of reactions is known as the thorium cycle. Nuclear reactors that use a cycle like this to produce fuel are called thermal breeder reactors. Thorium was discovered in 1828 by Jöns Jakob Berzelius but had few uses until the invention of the Welsbach mantle in 1885.